Radio antenna system



April 7, 1942. M. BRUCE RADIO ANTENNA SYSTEM Filed Ma rch 2i, 1941jwnzan M Cay,

Wzass Patented Apr. 7, 1942 STTS 10 Claims.

The present invention relates to radio antenna systems, and associatedtuning and coupling means, for the radiation and collection ofelectromagnetic waves. More specifically, the invention relates toantenna systems for vehicles, having particular though by no meansexclusive application to aircraft.

The problem of radio communication with vehicles especially two-waycommunication, has been complicated by difficulties in providing anadequate antenna system for the vehicle. This is particularly true inthe case of airplanes, where any external antenna, such as a trailingwire, exposed rods, or wires mounted on and insulated from the aircraftstructure, if of a size to be eifective, offers serious mechanicaldifficulties as well as materially reducing the speed of the craftthrough increasing its aerodynamic resistance.

In the present inventors prior United States Letters Patent No.2,235,139, issued March 18,

1941, there is disclosed an antenna system for aircraft which obviatesthe necessity of adding external elements such as wire, rods or thelike, to serve as the radio antenna. Instead, the metallic structure ofthe craft is employed as the antenna, one portion of the craft, such asan elevator or other pivotally mounted airfoil member, being insulatedfrom the remainder of the craft at supporting points, and the-mutuallyinsulated portions coupled to the radio equipment in a manner whichprovides a series tuned antenna system which may be operated over a widerange of frequencies substantially independent of the equivalentelectrical dimensions of the aircraft structure.

While the system of said application provides an effectiveself-contained antenna which is entirely satisfactory from theelectrical standpoint, the adjustment of the system generally being suchthat a current loop occurs in the vicinity of the point of coupling tothe structural portions of the craft, nevertheless in certain instancesthere may be mechanical difficulties in the way of insulating one ormore airfoil members attheir supporting points, especially where it isdesired to install the system in a craft already constructed in theconventional manner. The object of the present invention is therefore toprovide an improved antenna system for aircraft and other vehiclesemploying the metallic structure of the craft, or a portion thereof, asthe antenna, operation over a substantial frequency range being achievedwithout insulating or otherwise disturbing any structural portion of thecraft.

'More particularly, an object of the invention is to provide forvehicles a novel and improved antenna system employing the metallicstructure of the vehicle as the radiating and collecting portion of thesystem and operative as a resonant antenna system at substantially any,desired frequency of operation in no way.limited by the equivalentelectrical dimensions of the vehicle structure or portion thereofutilized as the antenna.

Another object of the invention is to provide a novel antenna couplingsystem particularly though by no means exclusively adapted for use inconjunction with vehicles the metallic structure of which is employed asthe radiating portion of the antenna system, whereby coupling may beeffectively provided to the said antenna system in the vicinity of avoltage loop thereon.

In accordance with these and other'objects, a feature of the inventioninvolves the provision of reactive means, preferably disposed adjacentone end of the vehicle and having at least its free or outer end exposedto space and unshielded by the metallic structure of the vehicle,wherein the vehicle structure and the reactive means comprise an antennasystem in which the vehicle structure constitutes substantially theentire radiating portion thereof, said reactive means being adjustableto provide values of reactance which enables the system as a whole, inrespect to its equivalent electrical length, to be made substantiallyequal to an even number of quarter-wavelengths of the selected operatingfrequency, and which likewise enables optimum coupling to the systemelement, for example a rod, and "an inductive member in series with saidcapacitive member. Through adjustment of the inductive member, the valveof the reactance may be varied over wide limits, enabling suitableadjustment of the antenna system to be attained for operation atfrequencies in no way limited by their relation to the equivalentelectrical length of the particular vehicle structure or portion thereofwhich is utilized as the effective radiating portion of the system.

For coupling the antenna system to the radio equipment ofthe vehicle,tuning and coupling means may be employed in series between the vehiclestructure and the external reactance, with the system adjusted in such amanner that a current loop appears within or in the vicinity of thecoupling unit when the antenna system is tuned to resonance. In case itis desired, howhaving the antenna system coupled to the inner orshielded conductor thereof in the vicinity of a voltage loop thereon andhaving the outer or shielding conductor short-circuited upon itself toprevent any appreciable shift in the voltage and current distribution onthe inner conductor through loading by the antenna. With such a couplingsystem, it becomes practical to couple to an antenna system in thevicinity of a-voltage loop thereon even though the radiating portion ofthe system may be of such character as to introduce a substantialloading effect, by reason of possessing relatively large values ofcapacitance per unit length, as for example, where the metallicstructure of an airplane or other vehicle is utilized as the radiatingportion of the antenna system.

In the accompanying drawing which illustrates the invention as appliedto aircraft, Figure 1 is a view showing in schematic fashion the tailportion of an airplane having an inductance and a capacitive membermounted at one end of the'fuselage structure in accordance with theinvention, with coupling to the antenna system provided in the vicinityof a current loop, and Figure 2 illustrates an alternative embodiment inwhich the coupling system of the invention is employed, with coupling tothe antenna system provided in the vicinity of a voltage loop thereon.

The fuselage of the airplane is indicated at 6, and it is contemplatedthat this will be either of all-metal construction,in accordance withgeneral practice in the larger craft, or having a metallic structure asin smaller airplanes, the invention being applicable to either type. Thedrawing also shows, merely for illustration, the rudder 8 and thevertical fin l0.

The radio equipment with which the craft is provided is indicatedgenerally at I2 and may be located at any convenient point in thecraft.The equipment may comprise high frequency apparatus such'as atransmitter or receiver for radio communication, direction findingapparatus, or other apparatus relating to the radiation or collection ofelectromagnetic waves. Such equipment is schematically indicated in thedrawing by the inductance M which may be taken to To enable the radioequipment to operate with maximum effectiveness over its entire. rangeof operating frequencies, it is necessary that a resonant antenna systembe provided. Since, in the case of aircraft or other vehicles noconnection to earth is available, the antenna system must have anequivalent electrical length such that a half-wavelength or multiplethereof of the operating frequency or, in other words, an even number ofquarter-wavelengths, may appear thereon.

In order that the metallic structure of the aircraft itself may beemployed as the radiating portion of a resonant antenna system operativeat frequencies whose half-wavelength or multiple thereof isnotequivalent to the electrical length of the portion of the aircraftstructure utilized,

there is provided in accordance with the invention means for alteringthe equivalent electrical length of the system as a whole, withoutrequiring as in the embodiment of the aforesaid application, electricaldiscontinuity in said structure through the use of insulation betweenstructural portions.

7 To this end there is employed at or adjacent one end of the vehiclestructure an adjustable reactance arranged so as to be at least in partexternal to and unshielded by the metallic structure of the vehicle.This reactance preferably consists of a capacitive member or area, whichmaybe in the form of a streamlined metal cap or shell I6, and anadjustable inductance l8 disposed within a housing 20 of insulatingmaterial secured at one end to the aircraft fuselage and supporting atits other end the shell IS. A lead 22 connects the outer end ofinductance to the shell. The reactance thus may be small in size andlight'in weight, and of a shape which will add negligible windresistance and hence will have but slight efiect on the speed of thecraft.

For operation of the structure of the craft as the radiating portion ofa resonant antenna system with coupling to the system provided in theVicinity of a current loop thereon, the arrangement of Figure 1 isemployed. In this embodiment the antenna tuning and coupling network,indicated generally at 24 and located as closely as possible to thepoint of connection to the antenna system, is connected in seriesbetween the end of the fuselage 6 and the inner end of the inductance I8. The tuningand coupling unit may comprise a variable reactance, hereillustrated as a parallel-connected inductance 26 and variable condenser28, coupled to the radio equipment by a transmission line 30 togetherwith coupling coils 32 and 34. A lead 36 from oneside of the tuning andcoupling reactance passes through an insulating bushing '38 to the innerend of the inductance I8, while the other side of the tuning andcoupling reactance is connected to the fuselage, preferably, in the caseof metal-clad structure, by passing the conductor through an opening inthe fuselage structure and connecting to the exterior surface as at 40,to facilitate the distribution of the radio frequency energy over theoutside surface of the craft.

For operation of the antenna system of Figure 1 at some selectedfrequency, adjustment of the reactance of the tuning and coupling unit24 will be coordinated with the value of the reactance of the externallymounted coil I8 and capacitive member l6 so as to provide resonantoperation with optimum coupling between the radio equipment and theantenna system. I This 'requlres that two conditions be complied with,

first, that the equivalent electrical length of the antenna system as awhole be substantially equal to an even number of quarter-wavelengths ofthe selected operating frequency, and second,

that at said frequency a current loop appear within or in the vicinityof the tuning and coupling unit 24. Since the equivalent electricallength of the effective radiating portion of the antenna system, thatis, the metallic structure of the vehicle, is fixed for any givenposition of the external reactance relative to said structure, theelectrical characteristics thereof cannot be altered. Accordingly, forany selected operating frequency, only a standing Wave or portionthereof app opriate to that frequency can appear on the vehiclestructure. The remainder of the standing wave, must therefore appear inthe tuning and coupling unit 24 and in the external reactance. At thesame time, the adjustment of the external reactance, as determined bythe tuning of inductance I8, must be such that a current loop is causedto appear within or in the vicinity of the coupling unit 24, in orderthat optimum coupling to the antenna system be achieved.

In other words, while there may be a variety of adjustments of externalreactance and tuning unit which together in conjunction with theequivalent electrical length of the metallic structure of the vehicle,comprise an antenna system of resonant length, nevertheless theseadjustments must be limited to such as will also cause a current loop toappear at or in the vicinity of the tuning and coupling unit 24. Sincethe external reactance may through suitable adjustment of inductance l8be varied over a wide range of values which may be made capacitive,inductive, or incertain cases purely resistive, the desired current andvoltage distribution in the tuning and coupling unit for optimumcoupling may readily be obtained while permitting the system as a wholeto be adjusted substantially to resonance. These adjustments, whilenecessary to permit operation in the desired manner, are neverthelessnot excessively critical in view of the relatively high capacitance perunit length of the radiating portion of the antenna system andconsequent rather broad resonance characteristics thereof, with theresult that exact.

resonance in tuning need not be achieved. As a result, the vehiclestructure may be made to serve effectively as the radiating andcollecting portion of an antenna system which is operative atfrequencies in no way limited by the equiv- {alent electrical length ofthe said vehicle strucure.

Where the operating frequency is such that the equivalent electricallength of the vehicle structure, for example, the fuselage of anairplane does not greatly exceed a quarter-wavelength or an odd multiplethereof, the antenna system may, through adjustment of the externalreactance and the tuning and coupling net-work, be tuned to resonancewith a half-wavelength appearing thereon. In cases where the equivalentelectrical length of the vehicle structure is such that the antennasystem, comprising the vehicle structure, the external resistance andthe tuning and coupling means, necessarily exceeds a half-wavelength ofthe operating frequency, then the system is preferably adjusted to beresonant at the next higher odd number of half-wavelengths in order thatthe antenna system may be terminated by portions having voltage loopsthereon which are opposite in phase. Alternatively, the antenna systemmay be arranged with the tuning and coupling net-work 24 exposed tospace, together with the external reactance, for example, by enclosingthe net work components in the insulating shell 20 together with theadjustable inductance it, such arrangement permitting operation. whenthe system is tuned to an even number of halfwavelengths of theoperating frequency as well as to an odd number of half-wavelengths.

In certain cases it may be found desirable to operate the antenna systemin the manner of an end fed antenna, with coupling to the antenna systembeing provided in the vicinity of a voltage loop thereon. For suchoperation the arrangement illustrated in Figure 2 may be employed toadvantage. In this embodiment of the invention, a coupling system isemployed which makes it possible to couple to a vehicle antenna system,comprising the vehicle structure and an external reactance, in thevicinity of a voltage loop on said antenna system, even though suchstructure generally has a substantial loading effect because of itsrelatively high capacitance per unit length. The coupling system is notlimited to such applications, however, being well adapted to couple toany continuous metallic structure for use of the said structure as aportion of an antenna system in instances where no ground is availableagainst which such structure may be operated. This results from the factthat any shift of the voltage distribution in the coupling system, withconsequent reduction of voltage in the vicinity of the coupling pointand corresponding decrease in coupling, is effectively prevented.

The embodiment of Figure 2 is similar to that of Figure 1 in that anexternal reactance is employed comprising an exposed capacitive area 42and an adjustable inductance 44, the inductance in this embodimenthowever being directly connected in series between the capacitive areaand the end of the fuselage 6 by connections 46. The external reactanceand the fuselage structure thus comprise an antenna system theequivalent electrical length of which may be varied by adjustment of theinductance 44. Depending upon the equivalent electrical length of thefuselage or such structural portion of the vehicle as may be employed asthe antenna, and the selected operating frequency, the externalreactance may, by suitable adjustment of the inductance, be madeinductive, capacitive, or in certain cases purely resistive, in orderthat the equivalent electrical length of the system as a whole may beequal to any even number of quarter-wavelengths of the operatingfrequency. As in the first described embodiment, it is desirable thatthe capacitive member be as large as convenient, having considerationfor weight, size, and other factors dependent on the character of thevehicle, in order that the size of the series inductance may becorrespondingly reduced, with consequent reduction of losses in theinductance. If, on the other hand, it is considered desirable for anyreason to reduce the size of the capacitive member to some small value,then the size of the series inductance may be correspondingly increased.

The coupling system by which the radio equipment 48 is coupled to theantenna system comprises a coupling unit which, for purposes ofillustration, is shown at a resonant quarter-wavelength shielded line,shorted at one end and having the shielding member short-circuited uponitself along its length, or in other words, electrically bonded acrosspoints normally of different potential to reduce said points to a commonpotential and thereby prevent the development of any standing waves onthe shielding member. Alternatively, a shielded line a quarterwavelength in length, or odd multiple thereof, may be employed, with theouter or shielding member short circuited to itself at its ends only, toprevent the development of standing waves thereon, and with one end ofthe inner or shielded conductor shorted to the shielding member. Thisresults in effectively maintaining the shorted end of the inner orshielded conductor at ground po tential under substantially allconditions of operation. Changes in voltage and current distribution onthe inner conductor, when an antenna or other load is connected thereto,are thus prevented,xso that uponcoupling an antenna to the innerconductor inthe vicinity of a voltage loop thereon, no appreciable shiftin fectively all portions of the shielding member, and particularly theadjacent inner surfaces '52 of the channel. Instead of being formed outof U-shaped channel or strip,the shielding member may for instance bestampedorotherwise-formed out of sheet metal to provide ahelically-grooved cylinder .similar to that formed by winding theU-shaped strip. The other portion 54 of the shielding member may be .ametallic shell or cylinder surrounding the portion to complete theshielding. The inner "or shielded element 56 of the line consists of aconductor disposed along the channel of the shielding member andinsulated therefrom along its length, except as appears hereinafter. Theconductor 56 thus has its adjacent turns electrically shielded from oneanother by the intervening partitions 58 formed by the sides of thegroove or channel in the shielding member 50, while the portion 54shields the conductor 56 from the surrounding metallic structure of thevehicle. The shielded conductor 56 is shorted at one end by jumper 60 tothe shielding portions 50 and 54, while the free end of the shieldedconductor is connected by a short and direct lead 62 through a couplingcondenser 64 to the capacitive area 42, which constitutes one end of theantenna system.

For a voltage loop to appear at the free end of the shielded conductor,the equivalent electrical length thereof from the shorted point to thefree end may be approximately equal to a quarter-wavelength of theoperating frequency or an odd multiple thereof. In certain cases,however, it may prove inconvenient or undesirable to employ in thecoupling system a line of substantially the proper resonant length.Accordingly, where a single line is to be utilized for operation of theantenna system over a range of frequencies, means are convenientlyemployed for adjusting the lineto resonance substantially independent ofits length. To this end, a reactive element, which is illustrated as avariable capacitance 66, may be connected across the free end of theline between the shielded and shielding member thereof, the value ofsaid reactance being such that the line may be tuned to resonance at theoperating frequency. Thus, in cases where the length of the line is lessthan one-quarter wave-length of the operating frequency, or falls shortof an odd multiple thereof by less than a quarter-wavelength, thereactive element must have a suitable value of capacitive reactance inorder for the line to be tuned to resonance and for a voltage loop toappear at the free end of the shielded conductor. If the'antenna systemis to be employed for communication at several different frequencieswith rapid shift from 'one to another, preferably more than one couplingsystem of the above type will be employed, each being of such a lengththat the line may be tuned to resonance over a particular band ofoperating frequencies.

The coupling unit may be coupled to the radio equipment by a transposedtransmission line 68, conductively coupled to the shielded. andshielding members of the coupling unit at points which will provide aproper impedance match between transmission line and coupling unit. Acoupling coil 10 inductively couples transmission line 68 to circuit 12representing the radio equipment.

There may be employed, in place of the above described coupling unit, aresonant concentric or coaxial line, having the antenna coupled to theinner or shielded member in the vicinity of a voltage loop, while theouter member or sheath is short-circuited to itself, for example, bybonding together points normally of different potential to prevent thedevelopment of any standing waves thereon. While such arrangement may besuccessfully employed in the embodiment of Figure 2 for operation in themanner of the invention, the coupling unit previously described isconsidered preferable in certain respects by'reason of the moreeffective short-circuiting of the inner surfaces of the shielding memberacross the relatively short path afforded to radio frequency voltagesbetween adjacent channels thereof.

While the invention has been particularly described as applicable toairplanes wherein the metallic structure of the craft is employed as theradiating portion of an antenna system, the system is equally,applicable to the collection of electromagnetic waves and accordinglyoperation of the antenna both for transmission and reception of saidelectromagnetic waves is contemplated throughout the specification. Itis likewise to be understood that the invention is in no way limited inits application to airplanes only, but may be employed in vehicles andcraft of other forms and types possessing electricallyconductivestructure.

The nature and scope of the invention having been indicated, and certainspecific embodiments thereof shown and described by way of illustration,what is claimed is:

1. In a vehicle having electrically conductive structure and providedwith radio equipment, a self-contained resonant antenna system utilizinga continuous portion of said structure as the antenna for operation atfrequencies substantially independent of the equivalent electricallength of the portion of the structure utilized, said system including areactance mounted adjacent one end of the vehicle structure andelectrically exposed to space at least in part, the reactance having avalue such that the equivalent electrical length of said system issubstantially equal to an even number of quarter-wavelengths of thedesired operating frequency, and means for coupling the antenna systemto the radio equipment.

2. In a vehicle having electrically conductive structure and providedwith radio equipment, a self-contained resonant antenna system utilizinga continuous portion of said structure as the antenna for operation atfrequencies substantially independent of the equivalent electricallength of the portion of the structure utilized, said system includingreactance mounted adjacent one end of the vehicle and electricallyexposed to space at least in part, the reactance comprising a capacitivemember and an inductive member connected in series therewith, thereactance having a value such that the equivalent electrical length ofthe system is substantially equal to an odd number of half-wavelengthsof the desired operating frequency, and means for coupling the radioequipment to the antenna system in the vicinity of a current loopthereon.

' 3. In a vehicle having electrically conductive structure and providedwith radio equipment, a self-contained resonant antenna system utilizinga continuous portion of said structure as the antenna for operation atfrequencies substantially independent of the equivalent electricallength of the portion of the structure utilized, said system including areactance mounted adjacent one end of the vehicle and electricallyexposed to space at least in part, the reactance comprising a capacitivemember and an inductive member connected in series therewith, thereactance having a value such that the equivalent electrical length ofthe system is substantially equal to an even number ofquarter-wavelengths of the desired operating frequency, and means forcoupling the radio equipment to the antenna system in the vicinity of avoltage loop thereon.

4. In a vehicle having electrically conductive structure and providedwith radio equipment, a self-contained resonant antenna system utilizinga continuous portion of said structure as the antenna for operation atfrequencies substantially independent of the equivalent electricallength of the portion of the structure utilized, said system including areactance mounted adjacent one end of the vehicle and electricallyexposed to space at least in part, the reactance comprising a capacitivemember and an inductive member connected in series therewith, tuning andcoupling means connected between the external reactance and the vehiclestructure and adapted to couple the radio equipment to the antennasystem, the external reactance and the tuning and coupling means havingvalues such that the total equivalent electrical length of the system isequal to an odd number of half-wavelengths of the operatingfrequency-and likewise such that a current loop appears in the vicinityof the tuning and coupling means.

5. In a vehicle having electrically conductive structure and providedwith radio equipment, a self-contained resonant antenna system utilizinga continuous portion of said structure as the antenna for operation atfrequencies substantially independent of the equivalent electrical.length of the portion of the structure utilized, said system including areactance disposed adjacent one end of the vehicle and comprising acapacitive member and an inductive member connected in series therewith,and tuning and coupling means connected between the reactance and thevehicle structure and adapted to couple the radio equipment to theantenna system, the reactance and said tuning and coupling means beingelectrically exposed to space and having values such that the totalequivalent electrical length of the system is equal to an even number ofquarterwavelengths of the operating frequency and like- Wise such that acurrent loop appears in the vicinity of the tuning and coupling means.

6. In a vehicle having electrically conductive structure and providedwith radio equipment, a self-contained resonant antenna system utilizinga continuous portion of said structure for operation at frequenciessubstantially independent of the equivalent electrical length of theportion of the structure utilized, said system including a reactancemounted adjacent one end of the vehicle and electrically exposed tospace at least in part, the reactance comprising a capacitive member andan inductive member connected between said capacitive member and thevehicle structure, the reactance having a value such that the equivalentelectrical length of the system is substantially equal to an even numberof quarter-Wavelengths of the desired operating frequency, and means forcoupling the radio equipment to the antenna system, said means includinga resonant shielded line comprising a shielded conductor and a shieldingmember, said line having the antenna coupled to the shielded conductorin the vicinity of a voltage loop thereon, the shielding member of theline being bonded across points normally of difierent potential toreduce said points to a common potential and thereby prevent theappearance of standing waves on said shielding member and likewiseprevent a shift in the voltage distribution on the shielded conductor.

'7. A coupling system for coupling radio equipment to an antenna system,comprising a resonant shielded line having a shielded conductor and ashielding member, said. line having the antenna system coupled to itsshielded conductor in the vicinity of a voltage loop thereon and havingthe shielding member bonded across points normally of differentpotential to reduce said points to a common potential, thereby toprevent the appearance of standing waves thereon and likewise to preventa shift in the voltage distribution on the shielded conductor.

8. A coupling system adapted to couple radio equipment to an antennasystem, comprising a shielding member, a conductor shielded thereby, anda shorting connection between the shielding member and the shieldedconductor, the shielding member being disposed with portions physicallyspaced along its length in direct electrically continuous relation toreduce the shielding member to a common potential at a plurality ofpoints along its length, thereby to prevent the appearance of standingwaves thereon and to minimize shifting of the voltage distribution ofthe shielded conductor upon coupling an antenna system to said shieldedconductor in the vicinity of a voltage loop thereon.

9. A resonant shielded line comprising a shielding member, a conductorshielded thereby, and a shorting connection between the shielding memberand the shielded conductor, the shielding member being bonded across aplurality of points to prevent the appearance of standing waves thereonand likewise to prevent a shift in the voltage distribution on theshielded conductor when a load is connected thereto.

10. A resonant shielded lineadapted to couple radio equipment to anelectrical load, comprising a conductor, a shielding member at leastpartially surrounding said conductor, and a shorting connection betweenshielding member and shielded conductor, the line having portions of theinterior shieldingsurface of the shielding member which lie oppositeportions of the shielded conductor spaced along said conductor, disposedin direct electrically connected relation to prevent the appearance ofstanding waves on said member and likewise to prevent a shift in thevoltage distribution on the shielded conductor when a load is connectedthereto.

MALCOLM BRUCE.

Patent No 2,279 ,150.

the same may conform to the record 0 MALCO LN BRUCE It is herebycertified that error appears in the printed specification tent requiringcorrection as follows: Page 5, first of the above numbered pa d--reactance--; and th t column, line 52, for the word "resistance" rea tshould be read with this correction therein that the said Letters Patenf the case in the Patent Office.

Signed and sealed this 19th day of'May, A. D. 1914.2.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.

